OLED (Organic Light-Emitting Diode) as a current type OLED is increasingly applied to high performance active matrix display devices. Conventional passive matrix OLED display devices need shorter driving time for a single pixel with increase of display size, and thus require increased instantaneous current; thus increasing the power consumption. At the same time, the application of large current causes too large voltage drop on the nano ITO (Indium Tin metal Oxide) line, and causes too large operating voltage of the OLED, reducing its efficiency. On the contrary, those issues can be well addressed in the AMOLED (Active Matrix OLED) display devices by inputting OLED current through line by line scan using switch transistors.
In backboard design of AMOLED, the main issue needing to be addressed is the non-uniformity in brightness between compensation circuits of AMOLED pixel units. AMOLED uses TFTs to construct a pixel circuit to provide corresponding driving current for the OLED. As known, low temperature poly-crystal silicon TFTs or oxide TFTs are mostly used. Compared with a general amorphous silicon TFT, the low temperature poly-crystal silicon TFT and the oxide TFT have larger mobility and more stable characteristics, and are more suitable for be applied to AMOLED display. However, due to the limitation of crystallization process, the low temperature poly-crystal silicon TFTs fabricated on a large glass substrate usually have non-uniformity on electrical parameters such as threshold voltage, mobility and so on. Such non-uniformity will be converted to driving circuit variation and brightness variation of the OLEDs which can be perceived by human eyes, i.e., the mura phenomenon.
Generally, for the AMOLED fabricated by the oxide TFT transistor process technology, the type of the OLED in the pixel circuit can be P type or N type. However, whether the P type or the N type TFTs are selected to realize the pixel circuit, the current flowing through the OLEDs not only changes with the variation of the turning-on voltage of the OLEDs due to long time stress, but also are different from each other due to the threshold voltage drift of the TFTs for driving the OLEDs. As a result, the brightness uniformity and the brightness invariance of the OLED display are affected as well.
Therefore, in order to address the above issues, it is needed to provide a pixel circuit and driving method thereof, and a TFT backboard.